It is known that valuable mineral deposits lie on the bottom of the ocean. Since nodules rich in manganese, for example, lie scattered on the ocean floor with a relatively low density, their direct collection without preparatory measures would not be economical. They are therefore first collected and then hauled to the surface. In the past, these two steps of the method, and also the type of equipment used in carrying out these steps, have always had one common feature, namely that the nodule collecting process on the sea bottom is directly coupled with the hauling process.
It is known from the trade journal "Ocean Industry," June, 1967, No. 6, at pages 37 to 39, to recover manganese nodules by means of two submerged units, whereby a ship tows a receptacle on the bottom of the sea which collects the nodules by sweeping the ocean bottom over a defined width, and a hauling unit disposed above the collecting receptacle and connected thereto. The nodules are collected in the receptacle, and are from there hauled upwardly by means of suction and by way of a hauling conduit connected between the end of the collecting receptacle and the hauling unit. Both steps of the method, namely collection and conveying upwardly, already pose considerable problems. In addition, the maximum possible rate of advance of the entire equipment does not permit a favorable adjustment between the two process steps, or only at an expenditure of such proportions that its success is questionable. There exists substantial uncertainty as to whether the economical object can be attained, namely to achieve a certain massive flow of the nodules on their upward path.
The production objective, the width of collection on the ocean floor and the rate of advance are related by the equation: EQU Q.sub.F = v .times. b .times. q,
wherein
Q.sub.f is the production objective (upward flow of solids or flow of mass collected) in kg/sec.; PA1 v is the rate of advance of the collecting device in m/sec.; PA1 b is the collecting width in m; PA1 q is the average nodule density on the ocean floor in kg/m.sup.2.
Assuming that the rate of collection amounts to 100% and that the amount of ore collected is also conveyed, the production objective at a given average nodule density can be influenced only through the speed "v" of the collecting device and the collecting width "b". More exact deliberations would have to be based on the fact that these rates can be achieved only asymptotically; however, this does not affect the basis of the present considerations.
The existing problems are brought to light by the following numerical example.
Although the economical production objectives fluctuate over broad ranges, one can reckon with an average production objective of 300 tons/hour, because this value corresponds approximately with the average production rate of a land-based ore mine.
With an assumed rate of advance of the collecting and conveying equipment of 0.5 m/sec. and an average nodule density of q = 10 kg/m.sup.2 (according to the latest exploration results) a collecting width of b = 16.6 m is required for the collecting device. If the possible rate of collection is reduced by 50%, the required collecting width would be double the above amount, namely 33 meters. In order to keep the required collecting width smaller, the above equation would require an increase of the rate of advance of the entire equipment; however, a rate of advance of 0.5 m/sec. is already decidedly too high.
The stationary hydrodynamic forces acting on the underwater equipment increase with the square of speed, or advancement; the additional problem posed by the hydroelastic vibrations need be merely mentioned here. The rate of advance has a particularly unfavorable effect on the hauling conduit, because the pipe segments must be equipped with buoyant bodies for compensating for the natural weight of the pipe segments. These buoyant bodies cause a substantial increase in flow resistance. This applies also to possible underwater stations provided within the pipeline assembly for the purpose of ore separation and for accommodating navigational and control equipment. It is impossible to determine exactly the additional resistive forces originating from the collecting equipment connected with the pipeline assembly as a result of an increased rate of advance, and the vibrational behavior of the pipeline.
Because of the afore-mentioned unfavorable influences, the speed of the entire underwater equipment should thus be kept in the order of magnitude of from 0.1 to 0.2 m/sec. However, with a collecting rate of 100%, this would entail collecting widths of from 45 to 90 meters.
This numerical example based on practical experience shows that the collection process should be carried out at a relatively high speed so that the collecting device may be kept within justifiable limits in terms of its dimensions, which is important for safe functioning of the device. On the other hand, the hauling of the nodules upwardly requires a relatively low rate of advance in view of the hydrostatic and hydrodynamic resistances. This relatively low rate of advance is economical only if the nodules are present in sufficient quantities. Therefore, the problem of nodule recovery lies in making such sufficient quantities available and in harmonizing such quantities with the rate of advance of the hauling equipment.